1. Field of the Invention
This invention relates generally to the on-line monitoring of a geometrical parameter of a continuously advancing elongated article and, more particularly, to a method of measuring and controlling the thickness of the pulp coating applied to pulp insulated wire.
2. Description of the Prior Art
There have been a number of techniques employed heretofore to measure the thickness of a coating material applied to continuously advancing articles, such as in the form of strip stock material or wire. Such techniques have often involved the use of radiation detectors, as well as mechanical, electro-mechanical and photo-electric devices to sense a change in the thickness of a coating applied to articles of the type in question.
With respect to radiation detectors, they necessarily have presented problems with respect to shielding because of their radioactive nature. Such detectors are also normally not optimally effective, and in certain cases may be totally ineffective, when employed to sense a change in a geometrical parameter, such as the diameter of an article having at least an outer coating in a partially wet, porous state (as distinguished from a hard, smooth and dry state).
As for mechanical and electro-mechanical sensing devices, they often are not capable of sensing or measuring extremely small physical changes in an article, particularly on a continuously monitored basis. This follows from the fact that the pressure switches and/or microswitches typically employed in such measuring apparatus are normally not sensitive to article geometric deviations falling in the range of one to several thousandths of an inch. Moreover, such switches generally have a limited switch contact life, unreliable repeatability over extended periods of use, and a relatively high inherent hysteresis, all of which contribute to the undesirability of using such measuring apparatus in on-line, high volume product manufacture.
As for the use of photo-electric sensors, they normally cannot readily measure geometrical variations in the order of one to several thousandths of an inch, particularly when such variations relate to the cross-section of an article having a curvilinear profile. Even for measuring larger variations, however, photo-electric sensors generally require rather expensive, fragile and complex arrays of miniaturized photo-electric diode matrices, together with complex wiring and logic circuitry to interface with such matrices.
Frequently, of course, the manufacturing area in which the geometrical parameter of an article is to be monitored also presents extreme environmental problems, such as with respect to shock, vibration, high temperature and/or humidity and dust. This often necessitates, in addition to convenience, that the actual processing of the sensor derived signal information take place at a remote location from the work functions being performed on the article. Such signal processing, for example, may include comparing the sensor initiated output signal with a reference signal, so as to determine the magnitude of any error, and thereafter producing a control signal for dynamically controlling some manufacturing process function. Unfortunately, when such low level generated output signals are electrical, and particularly when they comprise an analog representation of the sensed physical parameter (or condition) of an article, they are often subjected to considerable attenuation when transmitted over any appreciable distance. As a result, such low level signals do not always accurately represent the measured parameter because they have a tendency to develop non-linearities and are susceptible to externally induced noise.
A further disadvantage of many prior on-line measuring and control systems has been that they have required one or more interfaces to allow a given parameter, for example, to be sensed mechanically, the logic control functions involved in the measurement to be performed electrically, and the actuating of the control apparatus to be controlled either mechanically or pneumatically. When such multiple interfaces are required, not only is the control system generally more expensive and complex, but it normally requires considerably more space.
It is also appreciated, of course, that the nature of the article, itself, can often present problems with respect to the measuring of a given parameter (or condition) thereof. For example, with respect to transducers or sensors of the mechanical, electro-mechanical and radiation types, the following factors selectively have a bearing on the type of measuring system (including sensor) employed: cross-sectional profile, conductive (or insulative) characteristics, ferromagnetic properties, weight, working temperature, resiliency, and as previously mentioned, porosity and degree of wetness (or dryness), to mention but a few.
Because of the many aforementioned problems that may be selectively encountered in any given on-line measuring and control system, fluidic sensors, such as in the form of mating gauge plates with one or more grooved passageways extending therethrough, have been employed heretofore to sense or measure changes in the diameter or cross-section of an elongated article continuously advanced therethrough. The sensor head has a pre-established fluidic pressure applied thereto through an orifice that communicates with the passageway such that a certain back-pressure is established by the presence of an article when positioned within the passageway. The magnitude of the back-pressure is proportional to the article cross-section along any given discrete axial segment that extends across the internal orifice that communicates with the sensor head passageway.
Such sensor heads have been employed heretofore to continuously monitor variations in yarn denier and fabric porosity in the textile industry, for example, as well as having been proposed and/or used in various selective applications such as in measuring the diameter of finished product, such as wire, cable or other tubular articles of essentially symmetrical cross-section, as well as elongated articles of irregular cross-section, such as rope, braided strands, etc.
While fluidic sensor heads are advantageously immune to at least most environmental conditions, and are quite sensitive to extremely small geometric variations in the elongated article being monitored, they have been limited heretofore to on-line applications where the diameter (or the cross-sectional area) of the measured article passing therethrough does not vary appreciably from both a given nominal diameter (or cross-sectional area), and from the cross-sectional area of the grooved passageway formed in and extending through the sensor head. The reason for this is that should the article develop an abnormally large cross-sectional region along its length, such as due to a cobble (oversized joint or over build-up of material) on bare wire or continuously cast or extruded rod stock, or a glob (over build-up of insulation) on a coated wire, such imperfections could not always pass through the sensor head passageway, which is formed by mating, air-tight sealed gauge plates. As a result, either the rod stock or wire would break, or a jammed condition would develop whenever a cobble or glob would develop thereon. In either case, there would be a loss of production time required to re-string the wire (or rod) in an on-line manufacturing system. Other obstructions, such as knots in either single strand, or multiple twisted or braided strand, articles would likewise normally not pass through conventional fluidic sensor heads.
A specially designed mechanical apparatus for sensing the presence of cobbles formed on a continuously cast copper rod, together with a control system for interrupting further feed of the rod in response to a cobble, is disclosed in U.S. Pat. No. 3,552,161, of H. W. Garbe et al. In that apparatus, the cobble detector actually comprises a pair of scissor type arms, the lower ends of which are formed with semi-circular portions so as to define a substantially enclosing aperture through which the rod passes when of nominal diameter. The upper ends of the pivotally mounted arms are biased against spring-biased plungers of respectively associated microswitches. With such a cobble detector apparatus, the sensing arms necessarily must exert at least a slight tension on the rod whenever a cobble passes through the split aperture, as the pivotal arms must be displaced sufficiently not only to accommodate the cobble, but to actuate one or both of the microswitches.
While such increased tension is normally of no consequence with respect to cast rod stock or large gauge wire, such increased tension may very readily result in the breaking of fine gauge wire, of the order of 18 to 26 gauge typically used in pulp insulated cable.
As a possible alternative approach to obviating the aforementioned problem, there is an automated on-line system disclosed in S. Warsaw et al U.S. Pat. No. 3,172,779, for controlling the thickness of a coating applied to strip material, such as paper, which does not require any of the aforementioned types of geometric measuring sensors and, hence, would also not present any of the problems described above with respect to the sporadic development of globs on pulp coated wire if allowed to advance through the measuring system. In this last mentioned prior control system, relatively complex and expensive recording, signal generating and counting apparatus is employed to quantitatively determine the amount of coating material that is applied to a pre-calculated area of the strip stock. The calculated area is compared in a correlated manner with the measured length of strip stock that is actually advanced within a predetermined period of time. A generated control signal, representative of the difference between the correlated measured and calculated parameters, in accordance with one mode of operation of the system, is then fed to valve control apparatus for metering the amount of coating material that is applied to a given length of advanced strip stock.
Unfortunately, in the coating of small elongated articles, such as fine gauge insulated wire, a precise calculation of the surface area to be coated per unit length of the wire is normally not accurate in practice because of the tolerance variations in diameter that typically exist along the wire core. Moreover, when any change in the amount of coating material to be applied to the wire is only periodically determined, i.e., after a predetermined length of the wire has been advanced through the coating material, and the recorded length has been compared with the calculated area, not only that time delay, but the time delay thereafter to properly adjust the consistency of a pulp slurry, for example, would substantially rule out any possibility that the pulp coating thickness could be accurately controlled within stringent limits in an on-line, high volume system. Reference to "pulp" herein is intended to connote a dry wood pulp, comprised, for example, of sheets of coniferous woods such as spruce, jack pine, and hemlock. Such dry pulp is converted into a wood-based, fibre refined, pulp and water slurry suitable for subsequent use in the manufacture of insulated wire. For further details with respect to such pulp insulation, reference is made to an article entitled "Manufacturing Pulp Cable", appearing on pages 86-94 of the 1971 July-October Issue of The Western Electric Engineer.
In view of the foregoing problems, it is not surprising that with respect to one particular process relating to the manufacture of pulp insulated wire, one preferred technique for determining the pulp coating thickness heretofore has involved an off-line, time consuming method of cutting random sections of coated wire from a given reel thereof, and immersing the wire segments in a column of mercury, with the displacement of the latter being read on a suitable scale that is correlated with coated wire diameter. While such a method has been found to be very reliable, it not only has the aforementioned disadvantage of being very time consuming, but provides only random sampled measurements of the pulp coating thickness of otherwise continuously running, high volume manufactured product.
The seriousness of only periodically measuring the coating thickness of random coated wire segments, and the excessive time involved for doing so, may be best appreciated by briefly highlighting a typical pulp coating operation. In one particular pulp coating system, sixty laterally disposed wires are coated with pulp insulation at one time, on a continuous basis, and at a speed that typically may range from 150 to 200 feet per minute. It is thus seen that if a given sample wire segment should indicate a defective pulp coating on a given wire line, many hundreds, and often thousands, of feet of wire will have been produced before the defect is not only detected, but a correction made with respect thereto in the operating process. Such a correction typically involves changing the consistency of pulp in a pulp slurry confined within a vat through which the wires are drawn during the coating operation.
With a typical vat often containing one to three hundred gallons of pulp slurry, it may take as long as fifteen to twenty minutes to change the consistency of the pulp within the slurry sufficiently to increase or decrease the coating on the bare wires passed therethrough so that the coating thickness again falls within a predetermined range. Of course, in accordance with the above described random wire segment sampling and immersion technique, should the coating thickness not be adjusted sufficiently after the first defective sample (or samples) have been ascertained, the pulp consistency would again have to be adjusted with an additional delay, and the possible coating of many more hundreds (or thousands) of feet of wire that might ultimately have to be discarded as scrap.
There thus has been an urgent need for a reliable and accurate on-line system to measure the coating thickness of pulp insulated wire continuously and, thereby, to more responsively adjust the coating process, such as through very small changes in the concentration of pulp in the slurry thereof, so as to maintain the coating thickness variations within a very narrow range at all times. This is the only way that out-of-limits coating thickness, which may result in totally defective wire, can be eliminated in high volume manufacture.
Stringent control over coating thickness uniformity of pulp insulated wire is also extremely important if such wire is to produce minimal cross talk and attenuation when used in electrical communications equipment. This is particularly true in signal transmission applications requiring operating frequencies considerably above the voice range, as mutual capacitance and pair-to-pair unbalance effects, for example, progressively increase with frequency, and are directly dependent, in part, on both the thickness and uniformity of the wire insulation.
The aforementioned coating thickness-capacitance dependent parameter is often referred to in the trade as the Diameter over Dielectric (DOD), and is commonly defined as the thickness of the insulative coating, doubled, plus the diameter of the wire core, resulting in an overall insulated wire diameter measurement. When the DOD is maintained within very close tolerances, the capacitance effects of the wire are substantially minimized and remain relatively constant with length when employed as twisted pairs in multi-wire cables. It is thus seen that maintaining uniform coating thickness is very important in this regard.
Unfortunately, neither the aforementioned automated cobble detecting apparatus, regardless of the type of on-line measuring sensor that may be employed in conjunction therewith, nor the automated coating thickness control system described hereinabove that obviates the need for sensors, is readily adapted for use in accurately monitoring and controlling the coating thickness of relatively small diameter coated wire. Moreover, if such prior art apparatus were used in combination to monitor and control the coating thickness on a plurality of such wires, the apparatus would have to be duplicated for each wire to be monitored. This, of course, would be very expensive and require considerable space.
Accordingly, in connection with high volume multiwire pulp coating systems, there is also an urgent need for an on-line wire diameter measuring and coating thickness control system wherein a plurality of coated wires may be monitored simultaneously, but with a substantial portion of the control system being common to all of the monitored wires. To that end, it would be desirous to average the measured readings for all of the monitored coated wire diameters, and should one or more abnormal readings be obtained (such as due to a glob, bare wire segment or break), to remove such a reading at least momentarily from all of the other normal readings until the cause of the abnormal reading no longer existed, or was corrected. It would also be desirous to use fluidic sensor heads for monitoring the wires on-line in a manner that would obviate the aforementioned wire jamming or breaking condition should a glob of insulation be encountered. In this manner, the need to shut down the entire coating system would not be required until such drastic action was absolutely determined necessary.